I generally think that the sort of knowledge best learned with flash cards is probably knowledge not memorizing in the first place, with the truth of this being proportional to the size of the dataset being learned. For sets in the neighborhood of 200 or less, I don't think it's a big problem. Chemistry classes asking you to memorize the mapping of element names ⟷ element symbols seems okay. But memorizing the spelling of several hundred to thousands of words? That seems like a job for a machine.